Edexcel iGCSE Biology-4.18B Stages of Protein Synthesis- Study Notes- New Syllabus

Protein Synthesis – Transcription & Translation

🌱 Introduction

Proteins are made according to instructions in DNA.
This process is called protein synthesis and has two main stages:
Transcription → copying DNA into mRNA
Translation → assembling amino acids into a protein

1. Transcription (DNA → mRNA)

• DNA unwinds at the gene region.
• RNA polymerase reads the template strand of DNA.
• Complementary RNA nucleotides pair with DNA bases:
  • A → U (Uracil in RNA replaces T)
  • T → A
  • C → G
  • G → C
• A single-stranded mRNA is formed → carries genetic code from DNA.
• mRNA detaches & moves from nucleus → cytoplasm.

Key Points:

• mRNA = messenger RNA, temporary copy of a gene.
• Carries instructions in codons → groups of 3 bases coding for 1 amino acid.

2. Translation (mRNA → Protein)

• mRNA attaches to ribosome in cytoplasm.
• tRNA molecules bring specific amino acids to the ribosome.
• Each tRNA has an anticodon that pairs with complementary mRNA codon.
• Ribosome moves along mRNA → joins amino acids with peptide bonds in correct order.
• Stop codon reached → polypeptide folds into functional protein.

Key Points:

• Ribosome = site of protein synthesis.
• tRNA = carries amino acids, anticodon matches mRNA codon.
• Codon = 3-base sequence on mRNA coding for 1 amino acid.
• Anticodon = 3-base sequence on tRNA complementary to mRNA codon.

⚙️ Summary Table

📌 Quick Recap
– DNA → mRNA → Protein.
– Transcription = DNA copied to mRNA in nucleus.
– Translation = mRNA read at ribosome → tRNA brings amino acids → protein formed.
– Codons (mRNA) match anticodons (tRNA) → correct amino acid sequence.
Mnemonic: “DNA Transcribes, mRNA Travels, tRNA Translates into Protein.”